The invention relates to a method and device for calibrating the deviation of a received beam from its desired position in a terminal.
Communications between space terminals can be performed by means of optical data transmissions. For this purpose, the various optical units required for this are arranged on an optical bench and a telescope arrangement. For obtaining perfect data transmissions it is necessary that the optical axes of the units arranged on the optical bench take up predetermined, or respectively unvarying, relative desired positions, in general in a parallel orientation, and that tilting of these axes in respect to each other is prevented. Tilting manifests itself in particular in that the received beam does not arrive at its desired position, shows a deviation from this desired position. The reasons for such tilting of the axes of the optical units, which should be prevented, are, for example, the effects of heat and radiation, as well as aging and wear processes within the optical bench. To prevent or to minimize tilting, various steps are usually taken; in particular, the optical bench is designed to be as rigid as possible in order to prevent its bending, and the individual optical units are fixed in place on the optical bench only over a short area of their length, while they are unattached over the remaining length. An optical bench of this type is known from EP 0 844 473 A1, for example.
However, it has now been found that in spite of these steps having been taken, it is not possible in every case to keep the tilting of the optical axes of the optical units within very narrow tolerances in the required way. Moreover, it must be expected that in the future the thermal stresses will increase, on the one hand, which will lead to increasing tilting of the optical axes and, on the other hand, that the requirements regarding the accuracy of the precise mutual alignment of the optical axes will increase.
An additional problem occurs in the course of data transmissions between the mentioned terminal and a partner terminal, which is also located in space; the terminal and the partner terminal generally move in relation to each other. Therefore the transmitted beam must leave the terminal not in the direction of the position taken up by the partner terminal at that moment, but must deviate by a lead angle from this direction, taking the relative motion into account. To achieve this, a terminal usually contains a lead device and a lead sensor (point ahead sensor), i.e. a comparatively expensive device.
It is the object of the present invention to provide a method and a device for executing it, which will permit the optimal acquisition of the partner terminal, as well as perfect data transmissions, in spite of the usually occurring tilting of the optical axes of different optical units on an optical bench to be expected, as well as perfect data transmissions. It is the further object of the invention to propose a use of a preferred further development of the device of the invention.
In contrast to customary methods, wherein it was attempted to prevent a tilting of the optical axes of the individual optical units in relation to each other to the greatest extent possible, such tilting is tolerated in a novel way in accordance with the principle of the invention. But in order to be able to perform acquisition and data transmissions with perfect results in spite of this, it is necessary to detect this mutual tilting, or respectively the deviations of the axes of two optical units from their relative desired position, or respectively to calibrate it by determining the deviation of the received beam from its desired position. It is possible by means of this to take the effect of tilting on the data transmissions into account. An acquisition channel with an acquisition sensor, whose detection range is considerably greater than the detection range of the receiving channel, is used for the rough alignment of the terminal with its partner terminal. The deviation of the received beam from its desired position means that, in a fictitious projection of an image from the receiving sensor onto an isochronous image of the acquisition sensor, the received beam would be located not at the intended spot, for example in the center, of the image of the acquisition sensor. The mentioned calibration is understood to be the detection of the deviation of the received beam. A reflector unit is arranged for calibration. A small portion of the transmitted beam impinges on a reflecting surface of the reflector unit as the impinging beam and is reflected there as an outgoing beam, which reaches the acquisition channel and the receiving channel. An isochronous measurement is performed in the detection ranges of the acquisition sensor and the receiving sensor. The result of the measurement provides information regarding the deviation of the received beam from its intended position, or respectively provides the data required for the mentioned calibration. This device is simple, cost-effective and light and provides precise results, and this independently of whether the deviation is caused by a tilting of the optical axis of the transmitting or of the receiving channel. It is also advantageous that at least that portion of the deviation caused by tilting of the optical axis of the transmitting channel is doubled by the reflection in the reflector unit, so that a unequivocal signal is generated by this.
The installation of a lead sensor can be avoided in a simple way with the aid of an advantageous further development of the device of the invention; however, for this it is necessary for the reflector unit to have two further reflecting surfaces. The impinging beam is reflected as two further outgoing beams at these additional reflecting surfaces. The total of three reflecting surfaces are located obliquely in relation to each other in such a way that they do not have a common straight line, and their normal surface lines extend in different directions. The three beams constitute a triple beam, which can be employed for calibrating, or respectively correcting, the setting of the lead device. By means of this the arrangement of an additional channel for the lead device can be omitted.